Currently there are no separate properties for pos-flex, neg-flex or preferred size. If it doesn't change, there needs to be at least CSS OM access to the separate values. Parsing space-separated list is easier than functional notation, but figuring out the used value for preferred size is still far from trivial.

Proposal: have separate properties for flex components:
flex-positive: <number>
flex-negative: <number>
flex-preferred-size: <length>
This is the most simple and consistent way to address OM problem and I don't see any particular danger in separate cascading being possible, not much different from widtha and min-width being separate...